Clippy fixes

[notpeter]

Fix various clippy complaints.
No manual changes, these were all done by --fix.

If there's disagreement with any of the style changes (e.g. .front() vs .get(0)) let me know and I'm happy to apply the specific Clippy exceptions.

Commands

for runtime in \
    lua55 lua54 lua53 lua52 lua51 \
    luajit luajit52 \
    luau luau-jit luau-vector4 \
    luau-jit,luau-vector4
do
    cargo clippy --fix --allow-dirty --all-targets -j 1 \
        --no-default-features \
        --features "$runtime,vendored,async,send,serde,macros,anyhow,userdata-wrappers,serialize"
done

for runtime in \
    lua55 lua54 lua53 lua52 lua51 \
    luajit luajit52
do
    cargo clippy --fix --allow-dirty --all-targets -j 1 \
        --no-default-features \
        --features "$runtime,vendored,async,serde,macros,anyhow,userdata-wrappers,serialize"
done

cargo +nightly fmt